Air-gap forming techniques for interconnect structures

ABSTRACT

The present disclosure relates to a method of forming an interconnect structure. In some embodiments, the method is performed by forming a trench within a first dielectric layer and forming sacrificial spacers along sidewalls of the trench. The trench is filled with a conductive material, and the sacrificial spacers are removed after the trench has been filled with the conductive material. A second dielectric layer is formed over the first dielectric layer to leave an air-gap in a region from which the sacrificial spacers were removed.

REFERENCE TO RELATED APPLICATION

This Application is a Divisional of U.S. application Ser. No. 14/135,785filed on Dec. 20, 2013, the contents of which is hereby incorporated byreference in its entirety.

BACKGROUND

As dimensions and feature sizes of semiconductor integrated circuits(ICs) are scaled down, the density of the elements forming the ICs isincreased and the spacing between elements is reduced. Such spacingreductions are limited by light diffraction of photo-lithography, maskalignment, isolation and device performance among other factors. As thedistance between any two adjacent conductive features decreases, theresulting capacitance increases, which will increase power consumptionand time delay.

To reduce parasitic capacitance and correspondingly improve deviceperformance, IC designers utilize low-k dielectrics. One kind of low-kdielectric is produced by doping silicon oxide (SiO₂) with impurities.For example, while pure SiO₂ has a dielectric constant of 3.9,fluorinated silica glass in which SiO₂ has been doped with fluorine hasa dielectric constant of 3.5. Further, SiO₂ which has been doped withcarbon can have a dielectric constant that is further lowered to about3.0. Another kind of low-k material is produced by creating large voidsor pores in a dielectric. Voids can have a dielectric constant of nearly1, thereby reducing the dielectric constant of the porous material byincreasing the porosity of the material. Large pores, also referred toas air-gaps, can provide an extremely low-k dielectric between the twoconductive features.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a cross-sectional view of some embodiments of aninterconnect structure.

FIG. 1B illustrates a top-sectional view of some embodiments of aninterconnect structure.

FIG. 2 illustrates a cross-sectional view of some alternativeembodiments of an interconnect structure of a semiconductor device.

FIG. 3 illustrates a cross-sectional view of some alternativeembodiments of an interconnect structure.

FIG. 4 illustrates a flow diagram of some embodiments of a method offorming an air-gap for interconnect structures.

FIGS. 5A-14B illustrate some embodiments of cross-sectional views andcorresponding top-sectional views of a semiconductor substrate showing amethod of forming an air-gap for interconnect structures.

DETAILED DESCRIPTION

The description herein is made with reference to the drawings, whereinlike reference numerals are generally utilized to refer to like elementsthroughout, and wherein the various structures are not necessarily drawnto scale. In the following description, for purposes of explanation,numerous specific details are set forth in order to facilitateunderstanding. It may be evident, however, to one skilled in the art,that one or more aspects described herein may be practiced with a lesserdegree of these specific details. In other instances, known structuresand devices are shown in block diagram form to facilitate understanding.

The present disclosure relates to air-gap forming techniques. Usage ofair-gaps can reduce overall k-value, reduce capacitance, and improve RCdelay in Back-end of Line (BEOL) structures. By forming air-gaps onsidewalls of conductive bodies, such as metal lines, a k-value of thedielectric material is reduced. In some embodiments, a selective portionof dielectric material is etched away to form metal lines trenches.Prior to filling these trenches with a conductive material, sacrificialspacers are formed on the sidewalls of the trenches. Then a portion ofthe sacrificial spacers is removed including some portions that abut tosidewalls of up landing pads in the metal lines to which vias arecoupled upwardly. Sacrificial spacers locate at desired air-gappositions may be kept until the conductive material is filled. In suchways, there will be no air-gap abutting the sidewalls of the up landingpads, while air-gaps will be present on sidewalls for down landing pads.These techniques allow better via alignment while at the same timeproviding lower k-values. Damage and contamination to the dielectricmaterial between the metal lines introduced by the followed etching isalso reduced.

FIG. 1a illustrates a cross-sectional view of some embodiments of aninterconnect structure 100. A first low-k dielectric layer 104 is formedover a substrate 102. A first conductive layer 103 in the first low-kdielectric layer comprises a down landing pad 110 and an up landing pad114 that are arranged in a common horizontal plane 101. The down landingpad 110 has an air-gap 118 a between its sidewalls and the first low-kdielectric layer 104. Sidewalls of the up landing pad 114 are adjacentto the first low-k dielectric layer 104 without an air-gap being formed,shown by region 120 a. In some embodiments, the down landing pad 110 orthe up landing pad 114 can be connected to a third conductive layer (notshown) under the first low-k dielectric layer 104 or a device (notshown) disposed on the substrate 102.

Still in FIG. 1a , a second low-k dielectric layer 106 is disposed overthe first low-k dielectric layer 104. A first via 116 a is disposed inthe second low-k dielectric layer 106 and vertically aligned to the uplanding pad 114 in the first low-k dielectric layer 104. The first via116 a is disposed on the up landing pad 114 and connected to a secondconductive layer 108 over the second low-k dielectric layer 106. Thefirst low-k dielectric layer 104 and the second low-k dielectric layer106 can be porous material with some pores or voids in it.

FIG. 1b illustrates a top-sectional view of some embodiments of aninterconnect structure including an example top-sectional view of theinterconnect structure of FIG. 1a . A first metal line 124 of the firstconductive layer is disposed in the first low-k dielectric layer 104.The first metal line 124 comprises a first conductive body 113 and theup landing pad 114, which both lie in common horizontal plane 101. Thefirst conductive body has a first width W1 and the up landing pad 114has a second width W2 that is larger than the first width W1. The firstvia 116 a in the second low-k dielectric layer 106 is disposed on the uplanding pad 114. An air-gap 119 is adjacent to sidewalls of the firstconductive body 113. In some embodiments, the up landing pad 114 hasouter sidewalls that abut the first low-k dielectric layer 104. Noair-gap is formed between the outer sidewalls of the landing pad 114 andthe first low-k dielectric layer 104. The first metal line 124 canfurther comprise a down landing pad 126 from which another via 128descends. The down landing pad 126 has the same width, W1 as the firstconductive body 113 and has air-gaps 119 adjacent to its outersidewalls.

In some other embodiments, a second metal line 122 is arranged in thefirst low-k dielectric layer 104 in the common horizontal plane 101 withthe first metal line 124. The second metal line 122 comprises a secondconductive body 109 and a down landing pad 110, which have the samewidth W1. A second via 112 a is disposed under the down landing pad 110and is electrically coupled to a lower surface of the down landing pad110. An air-gap 118 a is adjacent to sidewalls of the down landing pad110 and extends continuously along sidewalls of the second conductivebody 109.

FIG. 2 illustrates a cross-sectional view of some alternativeembodiments of an interconnect structures, for example, 232, 234, 236 or238. Connection structures can be dual damascene structures (e.g., 238).A sidewall of connection structures can be perpendicular (232) or tilted(234), relative to a surface of the substrate. Air-gaps, such as 219 aand 219 b, can be disposed about sidewalls of down landing pads, such as239 a, 239 b, respectively. For up landing pads, such as 241, thesidewalls of the up landing pad 241 abut the surrounding low-kdielectric layer 204.

FIG. 3 illustrates a cross-sectional view of some alternativeembodiments of an interconnect structure. First and second low-kdielectric layers 304, 306 are formed between first and secondconductive layers 301, 308. The first conductive layer 301 can comprisea conductive device region, such as a source/drain region in asubstrate; or can comprise a metal interconnect layer or some otherconductive region. The second conductive layer 308 typically comprises ametal interconnect layer. A down landing pad 310 and an up landing pad314 are formed in the first low-k dielectric layer 304. A first via 316,which is vertically aligned to the up landing pad 314, is formed in asecond low-k dielectric layer 306 and electrically couples the uplanding pad 314 to the second conductive layer 308. The down landing pad310 is electrically coupled to the first conductive layer 301 through asecond via 320. An air-gap 318 is adjacent to sidewalls of the downlanding pad 310. In some embodiments, a third dielectric layer 322 canbe formed between the first low-k dielectric layer 304 and the secondlow-k dielectric layer 306. This third dielectric layer 322, can beformed, for example by chemical vapor deposition, and can have adifferent lattice structure from that of a low-k dielectric layer. Thisthird dielectric layer 322 can provide higher structural integrity tosupport the low-k dielectric layer. At least a significant portion ofthe air-gap 318 is left after formation of the third dielectric layer322. No significant air-gap is adjacent to sidewalls of the up landingpad 314 above which the first via 316 is connected to the secondconductive layer 308.

If we compare FIG. 1 to FIG. 3 (direct in FIG. 1), in some embodiments,the second low-k dielectric layer can be disposed directly on the firstlow-k dielectric layer. By avoiding a third dielectric layer with arelative high dielectric constant between the first low-k dielectriclayer and the second low-k dielectric layer, a lower overall effectivedielectric constant can be achieved.

FIG. 4 illustrates a flow diagram of some embodiments of a method offorming an air-gap for interconnect structures.

While disclosed methods (e.g., methods 400) are illustrated anddescribed below as a series of acts or events, it will be appreciatedthat the illustrated ordering of such acts or events are not to beinterpreted in a limiting sense. For example, some acts may occur indifferent orders and/or concurrently with other acts or events apartfrom those illustrated and/or described herein. In addition, not allillustrated acts may be required to implement one or more aspects orembodiments of the description herein. Further, one or more of the actsdepicted herein may be carried out in one or more separate acts and/orphases.

At 402, a selected portion of a first low-k dielectric layer is removedto form a first trench and a second trench. A photolithography processcan be applied for the opening patterning purpose.

At 404, a first set of sacrificial spacers adjacent to sidewalls of thefirst trench and a second set of sacrificial spacers adjacent tosidewalls of the second trench are formed. In some embodiments, asacrificial layer, for example, comprising Titanium Nitride (TiN) orTitanium Oxide (TiO), is firstly formed on a top surface of the firstlow-k dielectric layer and the sidewalls of the trenches. Then a portionof the sacrificial layer on a horizontal surface of the first low-kdielectric layer is etched away to form the sacrificial spacers.

At 406, the second set of sacrificial spacers is removed while the firstset of sacrificial spacers is kept in place to aid in air-gap formation.There would be no air-gap formed on the second sidewall of the secondtrench in the following process. In some embodiments, a mask is appliedto keep the second trench with sacrificial spacers open for etching. Anopening of the mask is wide relative to a horizontal dimension of theexpecting air-gap, thus alignment requirements are decreased.

At 408, a first barrier layer, for example, comprising Tantalum (Ta),Tantalum Nitride (TaN), Cobalt(Co) or their alloy is formed over theopening and the first low-k dielectric layer. The first barrier layercould be bilayer or multi-layer formed by more than one material.

At 410, a first via under the first trench is formed connecting to afirst device or a first conductive layer under the first low-kdielectric layer. The first via is connected to a first device or afirst conductive layer under the first low-k dielectric layer.

At 412, the first and second trenches are filled with a conductivematerial, for example copper, with the first set of sacrificial spacersin place in the first trench. A Chemical-Mechanical Polishing (CMP)process can be applied to smooth surfaces and remove a top portion ofthe conductive material that is not needed. Thus, a metal linecomprising a copper core and a barrier layer abutting outer lateralsidewalls of the core is formed.

At 414, a second barrier layer, for example, comprising Cobalt (Co) capand a silicide material are formed on an exposed surface of theconductive material. The second barrier layer can act as an etch stoplayer (ESL).

At 416, the second set of sacrificial spacers is removed after the firstand second trenches have been filled with the conductive material. Anopening for the air-gap is opened at this step.

At 418, a second low-k dielectric layer is formed over the first low-kdielectric layer to leave an air-gap in an region from which the firstset of sacrificial spacers were removed. It can be formed by spin-onprocess directly on the first low-k dielectric layer and the air-gap isformed. In some alternative embodiments, the second low-k dielectriclayer can be deposited after depositing another dielectric layer withrelative high k value by non-conformal CVD process.

At 420, a second via is formed in the second low-k dielectric layerabove the second trench connecting to a second device or a secondconductive layer above the second low-k dielectric layer. Since there isno air-gap on the sidewall of the second trench, there will be lessdamage to the first low-k dielectric layer during a process to open thesecond low-k dielectric layer downward to the second trench in the firstlow-k dielectric layer.

FIGS. 5a-14b illustrate some embodiments of cross-sectional views andcorresponding top-sectional views of a semiconductor substrate showing amethod of forming an air-gap for interconnect structures. Although FIGS.5a-14b are described in relation to method 400, it will be appreciatedthat the structures disclosed in FIGS. 5a-14b are not limited to such amethod.

As shown in FIG. 5a , a selected portion of a first low-k dielectriclayer 504 is removed to form a first trench 510 and a second trench 514.A mask layer 506 is applied for selective opening. FIG. 5b shows atop-view of the semiconductor substrate.

As shown in FIGS. 6a-6b , the mask layer 506 is removed.

As shown in FIGS. 7a-7b , a sacrificial layer 702 for example, TiN orTiO, with a thickness of from about 100 Å to about 500 Å is formed onthe first low-k dielectric layer 504.

As shown in FIGS. 8a-8b , a portion of the sacrificial layer on ahorizontal surface 802 of the first low-k dielectric layer 504 is etchedaway to form a first set of sacrificial spacers 818 and a second set ofsacrificial spacers 820.

As shown in FIGS. 9a-9b , the second set of the sacrificial spacers 820on sidewalls of the second trench 514 is removed with a mask 902 inplace. In some embodiments, a wet etching process comprising a wetetchant may be applied within a processing chamber held at a temperatureof between approximately 30° C. and approximately 70° C. For example,the wet etchant may comprise Hydrogen peroxide (H₂O₂) or Sulfuric Acid(H₂SO₄). A remaining portion of the sacrificial spacers, which includesa portion 818 on a first sidewall of the first trench and a portion 906on sidewalls of second trench, is kept for air-gap formation purposes.In some embodiments, a mask 902 is applied to keep the second trench 514with the second set of the sacrificial spacers 820 open for etching.Notably, an opening 904 of the mask 902 is wide relative to a horizontaldimension of the expecting air-gap, thus alignment requirements aredecreased.

As shown in FIGS. 10a-10b , a first barrier layer 1002, for example,comprising Ta, TaN, Cobalt (Co) or their alloy is formed over theopening and the first low-k dielectric layer. The first barrier layercould be bilayer or multi-layer formed by more than one material.

As shown in FIGS. 11a-11b , a first via 1112 under the first trench 510is formed. The first via 1112 is connected to a first device or a firstconductive layer under the first low-k dielectric layer 504.

As shown in FIGS. 12a-12b , a conductive material, for example, copper,is filled to the openings to form via 1112 a. A Chemical-MechanicalPolishing (CMP) process can be applied to smooth surfaces and remove atop part of the conductive material that is not needed.

As shown in FIGS. 13a-13b , in some embodiments, a second barrier layer1302, such as a Ta, TaN or Cobalt cap layer or a Co silicide material isformed to mitigate electromigration. In some embodiments, this secondbarrier layer 1302 can act as an etch stop layer (ESL) Then theremaining sacrificial spacer material is removed by a high selectiveetching process, for example, a wet etching at a temperature of betweenapproximately 30° C. and approximately 70° C. A wet etchant may compriseH₂O₂ or H₂SO₄. An opening for the air-gap is opened at this step.

As shown in FIGS. 14a-14b , a second low-k dielectric layer 1402 isformed over the first low-k dielectric layer 504. It can be formed byspin-on process directly on the first low-k dielectric layer 504 and theair-gap 1406 is formed. In some alternative embodiments, the secondlow-k dielectric layer 1304 can be deposited after depositing anotherdielectric layer (not shown) with relative high k value by non-conformalCVD process. A second via 1416 in the second low-k dielectric layer 1402is formed above the second trench. In some embodiments, the second via1316 is connected to a second device (not shown) or a second conductivelayer 1404 above the second low-k dielectric layer 1402. Since there isno air-gap on the sidewall of the second trench 514, there will be lessdamage to the first low-k dielectric layer 504 near the second trench514 during a process to open the second low-k dielectric layer 1402downward to the second trench 514 in the first low-k dielectric layer504.

It will be appreciated that while reference is made throughout thisdocument to exemplary structures in discussing aspects of methodologiesdescribed herein, that those methodologies are not to be limited by thecorresponding structures presented. Rather, the methodologies (andstructures) are to be considered independent of one another and able tostand alone and be practiced without regard to any of the particularaspects depicted in the FIGS. Additionally, layers described herein, canbe formed in any suitable manner, such as with spin on, sputtering,growth and/or deposition techniques, etc.

Also, equivalent alterations and/or modifications may occur to thoseskilled in the art based upon a reading and/or understanding of thespecification and annexed drawings. The disclosure herein includes allsuch modifications and alterations and is generally not intended to belimited thereby. For example, although the figures provided herein, areillustrated and described to have a particular doping type, it will beappreciated that alternative doping types may be utilized as will beappreciated by one of ordinary skill in the art.

In addition, while a particular feature or aspect may have beendisclosed with respect to only one of several implementations, suchfeature or aspect may be combined with one or more other features and/oraspects of other implementations as may be desired. Furthermore, to theextent that the terms “includes”, “having”, “has”, “with”, and/orvariants thereof are used herein, such terms are intended to beinclusive in meaning—like “comprising.” Also, “exemplary” is merelymeant to mean an example, rather than the best. It is also to beappreciated that features, layers and/or elements depicted herein areillustrated with particular dimensions and/or orientations relative toone another for purposes of simplicity and ease of understanding, andthat the actual dimensions and/or orientations may differ substantiallyfrom that illustrated herein.

In some embodiments, the present disclosure relates to a method offorming a metal interconnect structure. The method comprises forming atrench within a first dielectric layer and forming sacrificial spacersalong sidewalls of the trench. The method further comprises filling thetrench with a conductive material, and removing the sacrificial spacersafter the trench has been filled with the conductive material. Themethod further comprises forming a second dielectric layer over thefirst dielectric layer to leave an air-gap in a region from which thesacrificial spacers were removed.

In other embodiments, the present disclosure relates to a method offorming interconnect structures. The method comprises forming a firsttrench and a second trench within a first dielectric layer. The methodfurther comprises forming a first set of sacrificial spacers alongsidewalls of the first trench and a second set of sacrificial spacersalong sidewalls of the second trench. The method further comprisesremoving the second set of sacrificial spacers, and filling the firsttrench and the second trench with a conductive material with the firstset of sacrificial spacers in the first trench. The method furthercomprises removing the first set of sacrificial spacers after the firsttrench and the second trench have been filled with the conductivematerial, and forming a second dielectric layer over the firstdielectric layer to leave an air-gap in a region from which the firstset of sacrificial spacers were removed.

In yet other embodiments, the present disclosure relates to a method offorming an interconnect structure. The method comprises forming a trenchwithin a first dielectric layer, and forming sacrificial spacers alongsidewalls of the trench. The method further comprises removing thesacrificial spacers from a first part of the trench while keeping thesacrificial spacers in a second part of the trench. The method furthercomprises filling the trench with a conductive material, and removing aremainder of the sacrificial spacers from the second part of the trenchafter filling the trench with the conductive material. The methodfurther comprises forming a second dielectric layer over the firstdielectric layer to leave an air-gap in the second part of the trench.

What is claimed is:
 1. A method of forming a metal interconnectstructure, comprising: forming a trench within a first dielectric layer;depositing a sacrificial spacer material to form sacrificial spacersalong sidewalls of the trench; filling the trench with a conductivematerial; removing the sacrificial spacers after the trench has beenfilled with the conductive material; and forming a second dielectriclayer over the first dielectric layer to leave an air-gap in a regionfrom which the sacrificial spacers were removed.
 2. The method of claim1, further comprising: forming a via hole protruding outward from abottom surface of the trench between sacrificial spacers within thetrench; and filling the via hole with the conductive material beforeremoving the sacrificial spacers.
 3. The method of claim 2, furthercomprising: forming a barrier layer over the first dielectric layer andthe sacrificial spacers, wherein the via hole extends through thebarrier layer.
 4. The method of claim 3, wherein the air-gap has a firstsidewall comprising the barrier layer and an opposing, second sidewallcomprising the first dielectric layer.
 5. The method of claim 3, furthercomprising: performing a planarization process after filling the trenchand via hole with the conductive material to remove the barrier layerfrom over top surfaces of the first layer of dielectric material and thesacrificial spacers.
 6. The method of claim 3, wherein the barrier layercomprises one or more of: tantalum, tantalum nitride, and cobalt.
 7. Themethod of claim 2, further comprising: forming a masking layer over atop surface of the first dielectric layer and along sidewalls of thesacrificial spacers; patterning the masking layer to have an openingseparated from the sacrificial spacers by the masking layer; andselectively etching the first dielectric layer according to the openingto form the via hole.
 8. The method of claim 1, further comprising:depositing a layer of the sacrificial spacer material over the firstdielectric layer; and etching the layer of the sacrificial spacermaterial to remove the layer of the sacrificial spacer material fromhorizontal surfaces without completely removing the layer of thesacrificial spacer material from vertical surfaces.
 9. The method ofclaim 1, wherein the sacrificial spacers comprise titanium nitride ortitanium oxide.
 10. A method of forming interconnect structures,comprising: forming a first trench and a second trench within a firstdielectric layer; depositing a sacrificial material to form a first setof sacrificial spacers along sidewalls of the first trench and a secondset of sacrificial spacers along sidewalls of the second trench;removing the second set of sacrificial spacers; filling the first trenchand the second trench with a conductive material with the first set ofsacrificial spacers in the first trench; removing the first set ofsacrificial spacers after the first trench and the second trench havebeen filled with the conductive material; and forming a seconddielectric layer over the first dielectric layer to leave an air-gap ina region from which the first set of sacrificial spacers were removed.11. The method of claim 10, further comprising: forming a first via holeprotruding outward from a bottom surface of the first trench between thefirst set of sacrificial spacers in the first trench; and filling thefirst via hole with the conductive material.
 12. The method of claim 10,further comprising: forming a barrier layer after removing the secondset of sacrificial spacers; and forming a via hole extending through alower surface of the barrier layer.
 13. The method of claim 10, whereinthe first set of sacrificial spacers and the second set of sacrificialspacers comprise titanium nitride or titanium oxide.
 14. The method ofclaim 10, further comprising: performing a planarization process afterfilling the first trench and the second trench with the conductivematerial and before removing the first set of sacrificial spacers. 15.The method of claim 10, further comprising: forming a second via hole inthe second dielectric layer over the second trench; and filling thesecond via hole with a second conductive material.
 16. The method ofclaim 10, wherein the second set of sacrificial spacers are removedprior to filing the first trench and the second trench with theconductive material.
 17. A method of forming an interconnect structure,comprising: forming a trench within a first dielectric layer; formingsacrificial spacers by depositing a sacrificial material along sidewallsof the trench; removing the sacrificial spacers from a first part of thetrench while keeping the sacrificial spacers in a second part of thetrench; filling the trench with a conductive material; removing aremainder of the sacrificial spacers from the second part of the trenchafter filling the trench with the conductive material; and forming asecond dielectric layer over the first dielectric layer to leave anair-gap in the second part of the trench.
 18. The method of claim 17,further comprising: forming a barrier layer along a bottom surface ofthe trench and along sidewalls of the sacrificial spacers; and forming avia hole extending through the barrier layer along the bottom surface ofthe trench.
 19. The method of claim 18, wherein the barrier layer islaterally separated from the first dielectric layer by the air-gapwithin the first part of the trench, and wherein the barrier layerlaterally contacts the first dielectric layer within the second part ofthe trench.
 20. The method of claim 18, further comprising: forming avia hole protruding outward from a bottom surface of the trench betweensacrificial spacers within the trench; and filing the via hole with theconductive material prior to removing a remainder of the sacrificialspacers.